1. Field of the Invention
The present invention relates to a mouse model for spinal muscular atrophy, or SMA, and to methods of making and using said mouse model. Particularly, the invention pertains to a genetically engineered mouse model which genotypically and phenotypically mimics human patients with spinal muscular atrophy. Said mouse model is useful in developing and evaluating various methods for diagnosing and treating human SMA patients.
2. Description of the Related Art
Spinal muscular atrophy (SMA) is an autosomal recessive disease characterized by degeneration of anterior horn cells of the spinal cord leading to muscular paralysis with muscular atrophy. Clinical diagnosis of the disease is based on progressive symmetric weakness and atrophy of the proximal muscles. Affected individuals are usually classified into three groups according to the age of onset and progression of the disease. Type I SMA, also known as Werdnig-Hoffnan disease, is most severe, and affected children usually show SMA symptoms before 6 months of age and rarely live beyond the age of 2 years. Type II and Type III SMA are less severe, and the onset of symptoms varies between 6 months and 17 years of age [see references 1-4]. SMA is one of the most common fetal autosomal recessive diseases, with a carrier rate of 1-3% in the general population and an incidence of 1 in 10,000 newborns [see references 5 and 8].
The pathological abnormality of SMA is the loss of motor neurons in the anterior horn of the spinal cord and in the brain stem, but the brain cortex is usually unaffected [see reference 9]. All three types of SMA are characterized by a marked decrease in deep reflexes and a diffused symmetric weakness of proximal muscles. Electromyographic analyses show muscle denervation with neither sign of sensory denervation nor major change in the motor nerve conduction velocity. Muscle denervation with atrophic fibers has been confirmed by muscle biopsy. Fiber groupings show features of muscle immaturity, suggesting an arrest in development [see references 10 and 11].
Irrespective of clinical severity, molecular analysis has shown that both types of the disease, whether it has a severe early onset or a mild late onset, are linked to the same chromosome locus 5q13, suggesting genetic homogeneity [see references 12-15]. Positional cloning strategies have led to the identification of two candidate genes for SMA, namely, survival motor neuron (SMN) and neuronal apoptosis inhibitory protein (NAIP) [see references 16 and 17]. These two genes are positioned within a complex region that is duplicated on the long arm of chromosome 5, resulting in two copies of NAIP (i.e., NAIP.sup.T and NAIP.sup.C) and two copies of SMN (i.e., SMN.sup.T and SMN.sup.C) in the human genome [see reference 18]. Deletion of NAIP exon 5 was found in 22-60% of type I SMA patients and in 5-18% of type II and III patients [see references 19-22]. Such deletion is also found in 3% of normal individuals, suggesting the gene is non-essential for motor neuron survival. In contrast, 95% of SMA patients carry homozygous deletions of SMN.sup.T regardless of phenotypic severity [see references 16, 19-21]. Telomeric SMN (SMN.sup.T) and centromeric SMN (SMN.sup.C) differ only in five nucleotides, none of which results in an amino acid change. Rarely, asymptomatic individuals show homozygous SMN.sup.T deletion [see references 19, 22-24]. SMNC deletion was found in 2-5% of normal populations [see references 16, 22]. Several intragenic mutations have been found in SMA patients with specific disruption in SMN.sup.T, providing further evidence that SMN.sup.T is associated with SMA [see references 16, 18, 25-27]. The SMN.sup.C gene is also expressed in SMA patients, but expression pattern is different from that of SMN.sup.T. Most SMN.sup.C transcripts lack exons 3, 5 or 7, particularly 7, while most SMN.sup.T transcripts appear to be full-length mRNAs [see references 28-29]. Dosage analysis shows that the SMN protein is significantly decreased in Type I patients [see references 30-31].
The SMN gene shows no homology to any previously identified genes. Recently, the SMN protein has been identified as one of the heterogeneous ribonucleoprotein U-associated proteins. The subsequent immunohistochemical analysis localized the SMN protein in a specific nuclear structure called gems [see references 32-33]. Based on the association of the gems and coiled bodies, a role of the SMN protein in RNA metabolism has been suggested and confirmed in an in vitro analysis [see reference 34]. Because the SMN gene is ubiquitously expressed in neuronal and non-neuronal tissues [see reference 16], the mechanism involved in specific degeneration of motor neurons, a characteristic feature of SMA, remains to be elucidated.
In addition to the lack of understanding of the pathophysiology of spinal muscular atrophy, there is no specific treatment currently available for SMA patients. Although transgenic mouse models have been utilized in studying various human diseases, including development of methods for diagnosis and treatment, no one has taught or suggested that a useful mouse model can be built for spinal muscular atrophy by knocking out the native mouse Smn gene and then rescuing the mouse with human SMN.sup.C gene inserted into the mouse genome.
The applicant hereby discloses an invention of a mouse model for spinal muscular atrophy. Said mouse model is genotypically and phenotypically mimic to human SMA patients and can be used to understand the pathophysiology of SMA and to develop methods of diagnosis and treatment of the disease. Part of the present invention was published in "A mouse model for spinal muscular atrophy", Nature Genetics 24, 66-70 (2000), the entire content of which is expressly incorporated herein.